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I am well familiar with the concept of gasket. From my non-mechanical engineer perspective, I can understand how a standard rubber gasket works in various plumbing and automotive applications (such as O rings etc). However, working on both auto engines and residential plumbing over years I have noticed some fittings forming a sealed metal-to-metal connection without using a rubber gaskets. Some examples are galvanized or copper unions that look like this:

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In metal-to-metal connections like this that require a high pressure seal, what insures that small cracks in the connection (that a rubber O ring would be conforming to due to its soft texture) do not leak?

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amphibient
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2 Answers2

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The common pipe threads that are used in buildings for water and gas are tapered threads. The thread is cut on the cone rather than cylinder. In the US, these threads are called National Pipe Thread Tapered (or NPTT1, or simply NPT). They don't seal metal-to-metal. Thread seal tape is wound onto the male thread before it's screwed in. When the tapered threads bottom-out, they compress the tape, and it fills the gaps. Unlike a gasket or O-ring, the thread seal tape is good for only one mate. If the thread needs to be unscrewed, the old tape is cleaned out, and fresh tape is applied.

1 There is also National Pipe Thread Straight (NPTS), which uses a flat gasket.

2 There is a dozen major families of pipe threads. Some of them have a polymer gasket or O-ring, others don't. The trade-offs are between cost, ease of cutting thread along with other installation procedures, pressure and aggressiveness of the fluid in the pipe.

3 There are pipe connection systems that seal metal cone to metal cone. Swagelok, for example.

Nick Alexeev
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In addition to tapered threads for creating seals, there are Compression Fittings which use a matching male and female mating low angle taper to create a sealing surface. When properly torqued, they rely on elastic deformation to "squish" the mating surfaces together to create a seal. These are re-usable.

Crush washers are used in a similar fashion to create a seal. Instead of a taper, a softer metallic washer(usually aluminum or copper) is used between the male and female threads. When correctly torqued, the crush washer will plastically deform to create a seal between the fittings. When the connection needs to be serviced, usually the crush washers are replaced or annealed before re-assembly.

GisMofx
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